The present invention relates generally to materials and articles for use in handling molten metals and, more particularly, to refractory compositions and articles made therefrom which are resistant to the thermal shock and erosive effects of liquid steel.
In the continuous casting of steel it is common to employ slide gate valves to control the flow of the molten metal from a furnace or from a ladle to a tundish and thence from the tundish to the molds of the continuous caster. The refractory plates and nozzles of the slide gate valves are subjected to an extremely hostile working environment in which resistance to thermal shock and erosive chemical attack from the molten steel and slag are key attributes in achieving a reasonable refractory service life.
With the introduction of more chemically aggressive grades of steel, the ongoing goal to increase refractory life at a reasonable cost has also become an increasingly difficult challenge. Generally, refractory compositions may be formulated to provide greater resistance to chemical erosion but typically at the expense of lower thermal shock resistance. Conversely, thermal shock resistance properties may be increased but at the expense of diminished corrosion resistance.
Heretofore, a known refractory composition containing alumina-mullite has been used in the manufacture of some slide gate plates and nozzle combinations. This known alumina-mullite composition has been generally successful in resisting steel erosion, at least with conventional grades of steel, and, in addition, it has exhibited adequate thermal shock resistance for this application and at a reasonable cost.
It is also known to employ a carbon-bonded, alumina-graphite refractory composition as a slide gate material. This material, while exhibiting excellent thermal shock properties, nevertheless, suffers from the fact that the carbon component is subject to oxidation and subsequent erosion.
Alumina-zirconia compositions have also been proposed for use in slide gate plate applications as disclosed in U.S. Pat. Nos. 5,055,433 and 5,214,010, but these materials also contain substantial amounts of carbon to form a carbon bond in the refractory matrix. Carbon-bonded refractories are, of course, well-known for their thermal shock resistant properties. It is also conventional practice, as disclosed in U.S. Pat. No. 5,055,433, to fire or burn these carbon containing refractories at high temperatures, generally above 1000.degree. C. in a reducing atmosphere. Alternatively, it is also known, as taught in U.S. Pat. No. 5,214,010, to thermally cure a carbon containing pressed refractory shape at a temperature below the burning temperature and then impregnate the shape with tar, pitch or like hydrocarbon and bake. Tar impregnated slide gate plates of other refractory materials, such as, for example, magnesia and alumina are also known in the art.
The present invention overcomes the inherent expense and shortcomings of prior carbon-bonded slide gate refractories by providing an oxide-bonded refractory material which provides excellent erosion/corrosion resistance in the presence of chemically aggressive steel grades, such as, for example, calcium treated steels. The refractory composition of the present invention further provides superior thermal shock properties without the presence of carbon. The absence of carbon/graphite also lowers raw material costs and permits firing without special furnace atmospheres to protect the carbon. Thus, manufacturing costs are decreased to provide a less expensive product to the steel producer. As a result, overall steel production costs are lowered which translates into a lower per ton steel cost relative to the consumable refractory slide gate plates and components.